The use of liquid photopolymerizable resins for making relief printing plates is well known (see U.S. Pat. No. 3,556,791 to Suzuki et al.). Basically, the platemaking process involves spreading a uniform layer of liquid photopolymerizable resin on a substrate and exposing selected portions of the liquid resin layer to actinic radiation, whereby the exposed areas are hardened while the unexposed areas remain liquid. The plate is then subjected to a washing process which removes the unexposed, still liquid portions from the relief plate's surface.
An important variable in making high quality photopolymer relief printing plates is the viscosity of the resin. Low resin viscosity results in undesirable flow of the liquid photopolymer resin during the platemaking process. This resin flow can cause split lines and abnormal shoulders on half-tone dots in the finished plate. Also, to produce relief plates of consistent quality, it is important that the viscosity of the liquid photopolymerizable resin be uniform from batch to batch.
Increasing the resin viscosity decreases the incidence of flow-related printing defects. Several methods have been used to increase the viscosity of liquid photopolymerizable resins. Using a higher molecular weight, higher viscosity photopolymer is one obvious method. However, the higher molecular weight prepolymer is closer to its gel point than a lower viscosity prepolymer. Thus, close control during the prepolymer preparation is necessary or the entire reaction mixture may gel. Also, it is difficult to prepare a high viscosity prepolymer with good batch to batch uniformity.
Another way in which the resin viscosity can be controlled is by changing the resin formulation. By increasing the prepolymer concentration and/or decreasing the concentration of monomers which have an especially low viscosity, the overall viscosity of the resin can be increased. However, unless the desired viscosity increase is small (e.g., a few thousand cps.), large changes in the physical properties of the cured resin can result.